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Introduction to Bioelectricity Part II

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Introduction to Bioelectricity Part II Introduction to Bioelectricity Part II Sabato Santaniello Contributors: Dr. Brown, Dr. Kaputa, Dr. Kumavor, Dr. Shin (UConn BME dept.) An intuition of “biopotentials” Source: http://www.youtube.com/watch?v=8IFoUWb8kLQ&playnext=1&list=PL6D9E1BD5963BBF0C&feature=results_main 1 Biopotentials An electric voltage that is measured between points in a living cell, tissue, or organism, and which accompanies all biochemical processes Biopotentials An electric voltage that is measured between points in a living cell, tissue, or organism, and which accompanies all biochemical processes Retina Brain (ganglion cells and Biopotentials (neurons) photoreceptors) allows organs and muscles to communicate with each other Muscles (muscle fibers) Heart (cardiac cells) 2 Mechanisms behind biopotentials Dendrites (receive signals from other cells) Buttons (form links with other cells) Soma (cell’s life support center and information processing unit) Axon Myelin Myelin sheath (covers the cell and helps speed the impulses) Axon Soma: 0.01 – 0.05 mm (diameter) Axon: 0.001 – 1 m (length) 1 – 25 um (diameter) Mechanisms behind biopotentials Dendrites (receive signals from other cells) Buttons (form links with other cells) Soma (cell’s life support center and information processing unit) Axon Myelin Biopotentials are due to the occurrence of one or more electrical impulses (action potentials) at the level of single cells 3 Mechanisms behind biopotentials Dendrites (receive signals from other cells) Buttons (form links with other cells) Soma (cell’s life support center and information processing unit) Axon Myelin An action potential stems from ions moving across the membrane and travels down from the soma to the axon and buttons Mechanisms behind biopotentials Dendrites (receive signals from other cells) Buttons (form links with other cells) Soma (cell’s life support center and information processing unit) Axon Myelin V +++++ +++++ 4 Mechanisms behind biopotentials Dendrites (receive signals from other cells) Buttons (form links with other cells) Soma (cell’s life support center and information processing unit) Axon Myelin 50 0 action voltage(mV) -50 potential V +++++ -100 0 2 4 6 8 10 time (ms) +++++ Features of an action potential voltage threshold time Threshold The net excitation that a excitable cell receives must exceed a minimum intensity to generate an action potential 5 Features of an action potential all-or-none action potentials voltage threshold time Threshold The net excitation that a excitable cell receives must exceed a minimum intensity to generate an action potential All-or-none response An excitable cell responds to stimuli of increasing intensity by spiking more, but strength and speed of action potentials remain the same Features of an action potential all-or-none action potentials voltage threshold time Threshold The net excitation that a excitable cell receives must exceed a minimum intensity to generate an action potential All-or-none response An excitable cell responds to stimuli of increasing intensity by spiking more, but strength and speed of action potentials remain the same Intensity The shape of an action potential does not change along the cell’s axon 6 Ionic concentrations and channels To understand the mechanisms of an action potential let us first look at what happens across the cell’s membrane Ionic concentrations and channels charge separation across the cell membrane ++++++ ––––– Extracellular Intracellular Na+ (sodium) Gradients of ion K+ (potassium) concentration: Cl- (chloride) 7 Ionic concentrations and channels = − <0 ++++++ ––––– The inside of a cell is always more negative than the outside The transmembrane voltage at rest is typically between –100Extracellular mV and –60 mVIntracellular Na+ (sodium) Gradients of ion K+ (potassium) concentration: Cl- (chloride) Ionic concentrations and channels <0 ++++++ ––––– Potassium (K+) ion concentration is 30-50 times higher inside as compared to outside Across the membrane there are channels that let pass potassiumExtracellular ions only (ion-specificIntracellular channels) Na+ (sodium) Gradients of ion K+ (potassium) concentration: Cl- (chloride) 8 Ionic concentrations and channels <0 ++++++ ––––– = − Nernst equilibrium ≜ (channels are ion-specific) Extracellular Intracellular ≜ ion valence ≜ universal gas constant ≜ Faraday’s constant ≜ temperature (in ⁰K) Ionic concentrations and channels <0 ++++++ ––––– = − conductance Nernst equilibrium (reciprocal of ≜ (channels are ion-specific) resistance) Extracellular Intracellular ≜ ion valence ≜ universal gas constant ≜ Faraday’s constant ≜ temperature (in ⁰K) 9 Ionic concentrations and channels <0 ++++++ ––––– = − Nernst equilibrium ≜ (channels are ion-specific) Extracellular Intracellular −1 −1 = 1 = 8.31 J K mol = 5 mM −1 = 9.65×104 C mol = 37 ⁰C ≅ 310 ⁰K = 155 mM Ionic concentrations and channels <0 ++++++ ––––– = − Nernst equilibrium ≅−. (channels are ion-specific) Extracellular Intracellular −1 −1 = 1 = 8.31 J K mol = 5 mM −1 = 9.65×104 C mol = 37 ⁰C ≅ 310 ⁰K = 155 mM 10 Ionic concentrations and channels <0 ++++++ ––––– There are other channels across the membrane that let pass only sodium (Na+) or chloride (Cl-) ions Because ofExtracellular the concentration gradient,Intracellular the flow of Na+ and Cl- ions is opposite to Nathe+ (sodium) flow of K+ ions Gradients of ion K+ (potassium) concentration: Cl- (chloride) Ionic concentrations and channels <0 ++++++ ––––– = − = − Extracellular Intracellular = ≅ + = ≅ − 11 Goldman-Hodgkin-Katz equilibrium <0 ++++++ ––––– If nothing perturbs the cell, the sum of the ionic currents will eventually reaches zero (equilibrium). This happens when reaches the value + + = ≝ + + Goldman-Hodgkin-Katz equilibrium + + = ≝ + + is called Goldman-Hodgkin-Katz (GHK) equilibrium. It is approximately –85 mV , , and are permeability coefficients and account for the ability of the cell’s membrane to be crossed by potassium, sodium, and chloride ions, respectively 12 K-Na active pump The fact that + + = at GHK equilibrium does not mean that each ionic current is zero Extracellular Intracellular K-Na active pump There is still a flow of Na+, K+, and Cl- ions across the membrane due to concentration gradient Extracellular Intracellular 13 K-Na active pump Because the amount of ions inside a cell is finite, this flow would change the concentrations and hence the GHK equilibrium Extracellular Intracellular K-Na active pump The GHK equilibrium remains stable, instead, because there are active pumps across membrane Extracellular Intracellular 14 K-Na active pump A pump is a channel that actively pushes K+ ions inside the cell and Na+ ions outside the cell to maintain the original concentration gradient K+ K+ Na+ Na+ Extracellular Intracellular K-Na active pump A pump pushes 2 K+ ions inside the cell for every 3 Na+ ions removed from the cell K+ K+ Na+ Na+ Extracellular Intracellular 15 K-Na active pump A pump pushes 2 K+ ions inside the cell for every 3 Na+ ions removed from the cell Pumps change the electric balance between inside and outside until the concentrations of Na+ and K+ reach the original value again Pumps consume metabolic energy to perform this task Note this… = − = − = − = ⁄ + = ⁄ + = ⁄ + 16 Note this… = − = − = − = + = + = + Note this… = − = − = − outside pump pump − − − + + + inside 17 Note this… The cell’s membrane can be represented as a circuit and can be analyzed with tools we have learned! outside pump pump − − − + + + inside Note this… We just need to remember that resistances , , and may vary with the voltage i.e., Ohm’s law is not valid outside pump pump − − − + + + inside 18 Origins of an action potential (AP) Now we have enough tools to understand how an action potential begins. Let us first introduce three technical words: Origins of an action potential (AP) Now we have enough tools to understand how an action potential begins. Let us first introduce three technical words: Because it is more negative inside the cell than outside, the cell’s membrane is said polarized Depolarization: lessening the magnitude of cell polarization by making inside the cell less negative Hyperpolarization: increasing the magnitude of cell polarization by making inside the cell more negative 19 Origins of an action potential (AP) First, an action potential begins because chemical reactions occur at the dendrites and lead to a sudden increase of the voltage across membrane 50 0 (mV) threshold -50 -100 0 2 4 6 8 10 12 time (ms) Origins of an action potential (AP) During the phase in yellow this cascade of events happens: depolarization decrease of 50 0 (mV) threshold -50 -100 0 2 4 6 8 10 12 time (ms) 20 Origins of an action potential (AP) During the phase in yellow this cascade of events happens: depolarization + Na inflow decrease of 50 0 (mV) threshold -50 -100 0 2 4 6 8 10 12 time (ms) Origins of an action potential (AP) During the phase in yellow this cascade of events happens: depolarization + Na inflow decrease of 50 0 (mV) threshold -50 -100 0 2 4 6 8 10 12 time (ms) 21 Origins of an action potential (AP) During the phase in blue, a saturation has been reached and a new cascade of events starts: 50 0 (mV) threshold -50 -100 0 2 4 6 8 10 12 time (ms) Origins of an action potential (AP) During
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